This invention relates to a corrosion-resistant alloy for use in heat transfer tubes (boiler tubes) for heat-recovery boilers which are used in a high-temperature corrosive environment where chloride-containing fuel ash condensates are deposited on the surface of boiler tubes.
More particularly, the present invention is concerned with an austenitic high-Cr, high-Ni alloy which is particularly useful in a high-temperature corrosive environment and which is suitable for use in boiler heat transfer tubes such as superheater tubes, reheater tubes, evaporator tubes, and water-wall tubes for heat-recovery boilers installed in facilities for incinerating municipal refuse, industrial waste, sewage sludge, and the like (hereinafter referred to collectively as refuse) for energy recovery, black-liquor combustion boilers installed in paper factories, and other heat-recovery boilers.
Recently there has been much interest in utilizing energy of municipal refuse because it can take full advantage of the potential energy of waste materials. In fact, power generation by incinerating municipal refuse has already been performed in some municipal incinerators for internal use and for electricity supply to utilities. Also in the paper industry, black-liquor combustion boilers have been used for firing black liquor formed as a by-product in a pulping process in order to recover soda and generate electric power using the waste heat of combustion.
To maximize the efficiency of electricity generation in the above-described heat-recovery system, it is desirable to increase the temperature .and pressure of the steam. However, an increase in steam temperature results in an increase in the metal temperature of the boiler tubes, thereby accelerating corrosion of the tubes. An increase in steam pressure requires a material which has an improved high-temperature strength. Heat-recovery boilers presently used in municipal waste incinerators are predominantly those operated such that the metal temperature of superheater tubes is around 800.degree.-900.degree. F. However, it is expected that operating conditions with a higher metal temperature of superheater tubes which exceeds 900.degree. F. will be employed in the near future in heat-recovery boilers for refuse incinerators (hereinafter referred to as refuse-fired heat-recovery boilers), as is the case in black-liquor combustion boilers, in order to improve the power generation efficiency.
Since municipal refuse includes a large amount of plastics, the exhaust gas upon incineration of municipal refuse contains a considerable amount of hydrogen chloride. The fuel ash condensates (fuel slag in the form of fused salt) which are the residues of incineration also contain chloride compounds. Therefore, the metallic material of heat transfer tubes used in refuse-fired heat-recovery boilers suffers not only corrosion resulting from gaseous attack by hydrogen chloride but also corrosion induced by deposition thereon of chloride-containing fused fuel slags (so-called "hot corrosion"). These types of corrosion become serious problems in refuse-fired heat-recovery boilers. The same problems are also found in boiler tubes for black-liquor combustion boilers, since they are attacked by SO.sub.2 -containing combustion gases and chloride-containing fuel ash condensates, which are both corrosive.
Under the above-described circumstances, there is a need for a material for heat transfer tubes which has good high-temperature strength and improved corrosion resistance sufficient to withstand these severe corrosive environments at high temperatures.
Corrosion-resistant steels or alloys of austenitic phases which are known to have excellent high-temperature strength are desirable for use in high-temperature boiler tubes such as superheater tubes for heat-recovery boilers operated at high temperatures and high pressures.
Various materials of austenitic phases have been investigated in the United States for use in heat-recovery boiler tubes for municipal incinerators. For example, it is reported in Corrosion 87, Mar. 9-13, 1987, Paper No. 402 that tubes of Incoloy Alloy 825 (which corresponds to alloy NO8825 specified in ASTM B163 and B423) containing about 42% Ni, 22% Cr, and 3% Mo by weight were actually used as heat-recovery boiler tubes in a commercial municipal incinerator. According to that article, the high-Ni alloys exhibited improved corrosion resistance with minimum tube thinning caused by corrosion in high-temperature corrosive environments normally encountered in municipal incinerators in the United States.
Other articles dealing with corrosion of commercially-available conventional austenitic high-alloy steels in the above-described high-temperature corrosive environments include Corrosion 85, Mar. 25-29, 1985, Paper No. 12; Corrosion 89, Apr. 17-21, 1989, Papers Nos. 204, 206, 209, and 550; and P. Ganesan et al, Industrial Heating, December 1987, pp. 18-22. In "High-Temperature Corrosion of Tube Support and Attachment Materials for Refuse-Fired Boilers" by S. F. Chou et al, Proceedings of the 1985 ASME IEEE Power Generation Conference, Milwaukee, Oct. 20-24, 1985, various alloys including Incoloy Alloy 825 and Carpenter Alloy 20Cb-3 which contains 34.0% Ni, 2.5% Co, 20.0% Cr, and 2.0% Mo were tested for corrosion as a tube support and attachment material for refuse-fired heat-recovery boilers. These articles generally discuss uniform corrosion of austenitic high-alloy materials at very high temperatures in the range of 1100.degree.-1700.degree. F.
However, the maximum metal temperature of superheater tubes for refuse-fired heat-recovery boilers is estimated to be 1000.degree. F. at the highest. As described above, these boiler tubes are exposed to very severe corrosive conditions since they are attacked by chloride-containing fuel ash condensates deposited thereon in a hydrogen chloride-containing gas atmosphere. Therefore, it is necessary for such heat transfer tubes to have resistance not only to uniform corrosion but also to intergranular corrosion attack (which occurs preferentially at grain boundaries) in the temperature range of about 700.degree.-1000.degree. F. in the above-described environment. Furthermore, it is important that these tubes withstand stress corrosion cracking in such an environment, particularly in portions such as welded joints and bends where stresses are concentrated.
The present inventors investigated corrosion of various conventional austenitic alloys in the above-described high-temperature corrosion environment, including chloride-containing corrosive fused salts on the test materials as encountered in refuse-fired waste heater boilers and black-liquor combustion boilers. As a result, it was found that most of conventional high-Cr, high-Ni austenitic alloys such as Incoloy Alloy 825 have high susceptibility to stress corrosion cracking as well as uniform corrosion and intergranular corrosion under such conditions. Since boiler tubes are pressure vessels used at high temperatures and high pressures, it becomes a serious problem that conventional alloys are considerably susceptible to stress corrosion cracking in stress-concentrated portions such as weld joints and bends. This indicates a possibility of corrosion failure of tubes caused by stress corrosion cracking, and such a failure may lead to shutdown of an entire incineration plant. Therefore, it is important that a corrosion-resistant material for boiler tubes have good resistance to stress corrosion cracking.